Annular reactor testing and Raman surface characterization of the CPO of i-octane and n-octane on Rh based catalyst

In this work we report a kinetic investigation of the CPO of i-C8H18 (2,2,4-trimethylpentane) and n-C8H18 performed over a Rh/alpha-Al2O3 catalyst, focusing the attention on the tendency to form surface C-species. These two C-8 species were chosen as model components of liquid logistic fuels, with the same chemical formula but very different chemical structure. An isothermal annular reactor has been used to collect kinetically relevant data in a wide range of operating conditions and identify the prevalent kinetic dependences. A quantitative analysis of the data was performed by a 1D reactor model and a lumped molecular scheme was developed with rate expression associated with global reaction stoichiometries. The results show that the partial oxidation of i-C8H18 and n-C8H18 is an indirect process consisting of total oxidation and steam reforming reactions. For both i-C8H18 and n-C8H18 the total oxidation reaction occurs with first order dependence on the hydrocarbon concentration and a zero order dependence on the oxygen concentration, consistently with the assumption that the hydrocarbon activation on oxygen-saturated surface is the rate determining step of the process. In the case of steam reforming, both i-C8H18 and n-C8H18 show a weak dependence on the hydrocarbon concentration. The introduction of a C-poisoning term was necessary in order to describe the kinetic features observed and was estimated to be more important in the case of n-C8H18. The formation of surface carbon species during the CPO tests was revealed by TPO (Temperature Programmed Oxidation) measurements; they showed the presence of both highly reactive and less reactive C-species. In the case of CPO of n-C8H18 the abundance of less reactive C species was greater than in the case of i-C8H18. Raman measurements confirmed the formation of surface C species in the case of n-C8H18, while very weak C-signals were detected after the CPO of i-C8H18. (C) 2016 Elsevier B.V. All rights reserved.